In a previous study a novel reflex was described in subjects who had suffered spinal cord injury (SCI) at or cephalad to the sixth cervical root (C6; neurological level) and who had not recovered significant motor function below this level (Calancie, 1991). In response to electrical stimulation of the tibial and/or posterior tibial nerves in the lower extremity, short-latency motor responses were evoked form distal muscle groups in the upper extremity, particularly intrinsic hand muscles and wrist extensors. Conversely, subjects with clinically-identical injuries at the acute stage but who subsequently recovered some function below C6 did not demonstrate such responses, nor did able-bodied control subjects. The proposed study will investigate the origin of these reflexes, in that they might reflect either: 1) pre-existing neural circuitry which is ineffectual under ordinary circumstances but which becomes functionally possible reflecting aberrant CNS regenerative sprouting. Our preliminary studies indicate that these reflexes develop after the neural lesion, and are due to sprouting from axotomized lower extremity afferent axons which innervate motoneurons of the caudal cervical enlargement. If this is correct, we will have demonstrated a capacity for CNS plasticity previously unrecognized in the adult human spinal cord, and which may well bear upon future strategies to promote regeneration. Furthermore, based on our previous study that the development of such reflex activity bodes poorly for functional recovery, we will attempt to implement a therapeutic program in the acute SCI stage which is designed to minimize the development of these interlimb reflexes, which if successful may result in greater functional recovery than these individual experience on their own (which is almost none). Experiments will be conducted over a 5 year period, and will include subjects with acute and subacute (<1 year) injury (n= 50 subjects) and chronic (>1 year) cervical SCI (n = 100 subjects). We will make repeated measures on the same individuals. The discharge properties of upper extremity single motor units (SMU) recruited by various lower extremity stimuli (muscle twitch, tendon ta, light touch, heat, cold, hair pulls, skin vibration, electric shocks) will be a major focus of investigation. Properties of the sensory afferents and spinal cord long tracts mediating these reflexes will also be examined carefully. The extent and magnitude of upper extremity reflexes at different stages after SCI will be quantified as an indicator of whether or not the applied therapy is affecting spinal cord reflex organization. Similarly, a standardized test of muscle and sensation will be given to acute SCI research subjects to correlate these behavioral measures with the electrophysiological measures which we obtain.